Influence of Hydrothermal Pretreatment Temperature

processes

Article Influence of Hydrothermal Pretreatment Temperature on the Hydration Properties and Direct Carbonation Efficiency of Al-Rich Ladle Furnace Refining Slag

Yi Huang 1,* and Guo Xiong 2

1 School of Materials and Chemical Engineering, Hunan City University, Yiyang 413002, China 2 Hunan Hualin Xiangtan Iron and Steel Co., Ltd., Xiangtan 411101, China; [email protected] * Correspondence: [email protected]

Abstract: The influence of hydrothermal pretreatment temperature on the hydration products and carbonation efficiency of Al-rich LF slag was investigated. The results showed that the carbonation efficiency was strongly dependent on the morphology of hydration products and the hydration extent of the raw slag. Hydrothermal pretreatment at 20 ◦C or 80 ◦C favored the formation of flake-

shaped products with a higher speciﬁc surface area and therefore resulted in a higher CO2 uptake of 20 ◦C and 80 ◦C-pretreated slags (13.66 wt% and 10.82 wt%, respectively). However, hydrothermal pretreatment at 40 ◦C, 60 ◦C or 100 ◦C led to the rhombohedral-shaped calcite layer surrounding the unreacted core of the raw slag and the formation of fewer ﬂake-shaped products, resulting in ◦ ◦ ◦ a lower CO2 uptake of 40 C, 60 C and 100 C-pretreated slags (9.21 wt%, 9.83 wt%, and 6.84 wt%,

respectively).

Keywords: LF slag; hydrothermal pretreatment; temperature; hydration products; CO2 uptake Citation: Huang, Y.; Xiong, G. Influence of Hydrothermal Pretreatment Temperature on the Hydration Properties and Direct Carbonation Efficiency of Al-Rich 1. Introduction Ladle Furnace Refining Slag. Processes The growth of global greenhouse gas emissions was 2.0% in 2018 and there is no 2021, 9, 1458. https://doi.org/ sign that any of these emissions are peaking yet. The six largest emitters of greenhouse 10.3390/pr9081458 gases, together accounting for 62% globally, are China (26%), the United States (13%), the European Union (more than 8%), India (7%), the Russian Federation (5%), and Japan Academic Editor: Federica Raganati (almost 3%) [1]. China’s carbon emission peak is a matter of international focus. Recently, China made a solemn promise to peak its carbon dioxide emission by 2030 and achieve Received: 29 July 2021 carbon neutrality by 2060. Some of the main measures China will use to reduce CO2 Accepted: 19 August 2021 emissions over next 10 years are: changing energy and industrial structures, transforming Published: 21 August 2021 the development mode, promoting clean energy, and appropriately increasing carbon sequestration ability. Among the current CO2 sequestration routes, mineral carbonation is Publisher’s Note: MDPI stays neutral regarded as a potential technology because of its advantages; it is environmentally benign, with regard to jurisdictional claims in it enables the permanent trapping of CO2 in the form of carbonate, and it does not require published maps and institutional affil- post-storage surveillance for CO leakage [2]. In general, mineral carbonation can be iations. 2 divided into two categories, namely direct carbonation and indirect carbonation. Direct mineral carbonation is accomplished through the reaction of a solid alkaline mineral with CO2, either in gaseous or in aqueous phase [3]. Alkaline solid wastes such as red mud, steel slag, blast furnace slag, fly ash, etc., are Copyright: © 2021 by the authors. used for direct mineral carbonation as efficient and economically available capturers of Licensee MDPI, Basel, Switzerland. CO2 [4–9]. For the direct mineral carbonation of steel slag, the formation of an increasingly This article is an open access article thick and dense carbonate layer surrounding the unreacted core of the solid particle hinders distributed under the terms and further carbonation and results in the lower CO capture capacity [10]. In our previous conditions of the Creative Commons 2 study [11], the improvement in the direct carbonation efficiency of Al-rich ladle furnace Attribution (CC BY) license (https:// refining slag (LF slag) by hydrothermal pretreatment was investigated. The results showed creativecommons.org/licenses/by/ ◦ 4.0/). that after hydrothermal pretreatment at 80 C, the morphology of Ca12Al14O33(C12A7) in

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the slag transformed from separated particles to the flake-shaped Ca3Al2O6·xH2O(C3AHx), resulting in an increased reaction surface area and carbonation efficiency. However, this study did not discuss the effect of hydrothermal temperature on the carbonation efficiency. In fact, the hydration product of C12A7 is dependent on the hydration temperature. ◦ Koplík et al. [12] reported that at 20 C the major hydration products of C12A7 were ◦ Ca2Al2O5·8H2O(C2AH8) and CaAl2O4·10H2O(CAH10); at 30 C CAH10 disappeared and ◦ only C2AH8 remained; at 60 C the only stable hydrates–Ca3Al2O6·6H2O(C3AH6) and Al(OH)3(AH) were formed. Edmonds et al. [13] stated that both C2AH8 and CAH10 can ◦ be produced during the hydration of C12A7 at 4 C, while no trace of CAH10 was spotted ◦ when C12A7 was hydrated at 20 or 40 C. Given that the morphology of the hydration product of C12A7 has a significant effect on the carbonation efficiency of LF slag and the type of hydration product produced is related to temperature, the aim of this study was to investigate the influence of hydrothermal pre-treatment temperature on the hydration properties and the carbonation efficiency of Al-rich LF slag at ambient temperature and pressure. Moreover, the relation between the morphology of the hydration product and the carbonation efficiency was clarified in this work.

2. Materials and Methods 2.1. Materials The Al-rich LF slag used in this study was collected from the Xiangtan steel plant in Hunan province, China. The chemical composition of the slag determined by X-ray Fluorescence (XRF) is listed in Table1. Before pretreatment, the raw slag was crushed and ground into a powder <20 mesh particle size. Distilled water was used in this study for slag suspension preparation.

Table 1. Chemical composition of raw slag as determined by XRF analysis.

Component CaO Al2O3 SiO2 MgO TiO2 SO3 Fe2O3 Others wt% 52.0056 23.66 15.83 4.05 0.90 2.45 0.54 0.57

2.2. Hydrothermal Pretreatment of LF Slag at Different Temperatures At first, the raw slag was fully mixed with water at a solid/water (S/W) ratio of 1:10 in a beaker. Then, the suspension was stirred for 30 min at 20, 40, 60, 80 and 100 ◦C (designated as 20H, 40H, 60H, 80H and 100H-slag, respectively). Next, the suspension was filtered and the obtained solid was sufficiently washed and dried to a constant weight at 105 ◦C for further characterization and for the following carbonation experiment.

2.3. Direct Aqueous Carbonation Process The schematic diagram of the aqueous carbonation experimental system is shown in Figure1. The pretreated slag suspension with a solid/water ratio of 1:10 was placed in a conical flask into an electric-heated thermostatic water bath equipped with a mechanical ◦ stirrer. The temperature of the water bath was kept at 40 C. Then, 99.99% pure CO2 from the CO2 cylinder was injected into the suspension at a flow rate of 5 L/min controlled by a flowmeter, and was simultaneously stirred for carbonation. The suspension underwent the carbonation process for 60 min and was then filtered. The obtained solid was dried to a constant weight at 105 ◦C for further characterization. Processes 2021, 9, 1458 3 of 12 Processes 2021, 9, 1458 3 of 12

FigureFigure 1. 1. SchematicSchematic diagram diagram of of the the aqueous aqueous carbonation carbonation experimental experimental system system..

2.4.2.4. Characterization Characterization of of Slag Slag X-rayX-ray diffraction (XRD,(XRD, BrukerBruker AXS AXS company company D8 D8 Advance, Advance, Karlsruhe, Germany) Germany) was con- was ductedconducted on the on slags the slags to identify to identify their theirmain main mineral mineral phases. phases. The scanning The scanning range rangewas from was ◦ ◦ ◦ 5°from to 70° 5 to2θ 70 at 2°/min.2θ at 2 TG-DSC/min. TG-DSC analysis analysis was performed was performed using a using METTLER a METTLER TOLEDO TOLEDO 1600 LF1600 thermal LF thermal gravimetric gravimetric analyzer. analyzer. A field-emis A field-emissionsion scanning scanning electron microscope electron microscope (FESEM, Hitachi(FESEM, company Hitachi companySU8010, Japan) SU8010, was Tokyo, used Japan)to characterize was used the to morphology characterize theof the morphology slag. The specificof the slag. surface The area specific of the surface slags areawas ofmeasured the slags by was the measured N2 gas adsorption by the N2 Brunauer–Em-gas adsorption met–TellerBrunauer–Emmet–Teller (BET) method (ASAP (BET) method 2020, Micromeritics, (ASAP 2020, USA). Micromeritics, The pH value Norcross, of the GA, slag USA). sus- pensionThe pH was value determined of the slag by suspension a PHS-3C was pH determinedmeter. by a PHS-3C pH meter. 2.5. Analysis of Carbonation Efficiency 2.5. Analysis of Carbonation Efficiency In order to compare the carbonation efficiency of Al-rich LF slags under the hydrother- In order to compare the carbonation efficiency of Al-rich LF slags under the hydro- mal pretreatment at different temperatures, the CO2 uptake of the slags was measured thermal pretreatment at different temperatures, the CO2 uptake of the slags was measured based on the weight fraction of the TG curve (∆m600–800 ◦C) and the dry weight (m) [14] based on the weight fraction of the TG curve (Δm600–800 °C) and the dry weight (m) [14] expressed in terms of CO2 (wt%), Equation (1): expressed in terms of CO2 (wt%), Equation (1): ∆m ◦ Δm600–800600–800°CC COCO2(wt%(wt%)=) = ×100×100 (1) (1) 2 mm 3. Results and Discussion 3.3.1. Results Influence and of Discussion Hydrothermal Temperature on Hydration Properties of Al-Rich LF Slag 3.1. InfluenceFigure2 of shows Hydrothermal the XRD Temperature patterns of on the Hydration raw slag andProperties the pretreated of Al-Rich slags. LF Slag The main mineralFigure phases 2 shows of raw the slag XRD were patterns C12A7 andof the Ca raw2SiO 4slag(CS 2and). For the all pretreated the slags with slags. pretreatment, The main mineralthe peaks phases of C12 ofA 7rawwere slag reduced, were C indicating12A7 and itsCa hydration.2SiO4(CS2). TheFor 40H-slagall the slags and with 100H-slag pretreat- pre- ment,sented the relatively peaks of more C12A intense7 were residual reduced, C12 indicatingA7 peaks thanits hydration. the otherslags, The 40H-slag suggesting and the 100H- lower · · · slaghydration presented extent relatively of these twomore slags. intense For residual the 20H-slag, C12A 3CaO7 peaksAl than2O3 CaCOthe other3 11H slags,2O(C suggest-4ACH11) ingwas thethe dominantlower hydration hydration product.extent of A smallthese amounttwo slags. of (C4 AForCH 11)the also 20H-slag, appeared in the 40H-slag and C AH was the other main product for this slag. Hydrocalumite 3CaO·Al2O3·CaCO3·11H32O(C64ACഥ H11) was the dominant hydration product. A small (Ca Al (OH) CO ·5H O) was only present in the 100H-slag, and this slag had the most amount4 2 of (C412ACഥH311) also2 appeared in the 40H-slag and C3AH6 was the other main prod- intense C AH peaks. With respect to the 60H-slag, only C AH crystal was found. In uct for this3 slag.6 Hydrocalumite (Ca4Al2(OH)12CO3·5H2O) was3 only6 present in the 100H- general, C3AHx, C4ACH11, and C3AH6 were the main hydration products, while the other slag, and this slag had the most intense C3AH6 peaks. With respect to the 60H-slag, only products mentioned in the “Introduction” (such as CAH10 and C2AH8) were not observed. C3AH6 crystal was found. In general, C3AHx, C4ACഥH11 , and C3AH6 were the main hydra- This could be explained by the fact that CAH10 and C2AH8 are the transition phases and tion products, while the other products mentioned in the “Introduction” (such as CAH10 can be converted to the ultimate stable products (e.g., C3AH6 and C3AHx), described by and C2AH8) were not observed. This could be explained by the fact that CAH10 and C2AH8 Equations (2)–(4), respectively [15]. are the transition phases and can be converted to the ultimate stable products (e.g., C3AH6 and C3AHx), described by Equations (2)–(4), respectively [15]. 2CAH10 → C2AH8 + AH3 + 9H (2)

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Processes 2021, 9, 1458 4 of 12 2CAH10 → C2AH8 + AH3 + 9H (2)

3C2AH8 → 2C3AH6 + AH3 + 9H (3) 3C2AH8 → 2C3AH6 + AH3 + 9H (3) 3C2AH8 → 2C3AHx + AH3 + (21 − 2x)H (4) 3C2AH8 → 2C3AHx + AH3 + (21 − 2x)H (4)

Figure 2. XRD patterns of (1) raw slag, (2) 20H-slag, (3) 40H-slag, (4) 60H-slag, (5) 80H-slag, and Figure(6) 100H-slag. 2. XRD patterns of (1) raw slag, (2) 20H-slag, (3) 40H-slag, (4) 60H-slag, (5) 80H-slag, and (6) 100H-slag. Figure3 displays the FESEM pictures of the raw slag and the pretreated slags. The raw slagFigure appeared 3 displays as irregular-shaped the FESEM pictures particles of withthe raw dense slag and and coarse the pretreated surfaces (Figureslags. The3a). rawAfter slag hydration appeared at as 20 irregular-shaped◦C, the slag surface particles became with smooth dense dueand tocoarse the formation surfaces (Figure of flake- 3a).shaped After Chydration4ACH11 (Figure at 20 °C,3b). the In slag addition, surface metastablebecame smooth hydrates due to in the the formation form of hexagonal of flake- shapedplatelets C4AC [14ഥ]H were11 (Figure observed 3b). In (Figure addition,3b). metastable The microstructure hydrates ofin thethe 40H-slagform of hexagonal presented plateletsas a mixture [14] were of C 4observedACH11, metastable (Figure 3b). hydrated The microstructure hexagonal-shaped of the 40H-slag platelets presented [16], and as un- a hydratedmixture of slag C4AC particlesഥH11, metastable (Figure3c). hydrated The edge hexagonal-shaped of the unhydrated platelets particles [16], in the and 40H-slag unhy- dratedwas covered slag particles by rhombohedral-shaped (Figure 3c). The edge CaCO of the3 unhydrated(calcite) particles particles and in AH the gel 40H-slag with a grainwas coveredsize of 0.5by µrhombohedral-shapedm, which may hinder CaCO the further3 (calcite) hydration particles of Cand12A AH7 (Figure gel with3d). a Once grain again, size ofthis 0.5 verifiedμm, which that may the hinder thick andthe further dense CaCOhydration3 layer of C surrounding12A7 (Figure the3d). unreacted Once again, core this of verifiedthe solid that particle the thick was and the dense main causeCaCO of3 layer the low surrounding carbonation the efficiency unreacted of core the slagof the without solid particlehydrothermal was the pretreatment,main cause of asthe illustrated low carbon ination our previousefficiency studies of the slag [9]. without CaCO3 andhydro- AH thermalgel should pretreatment, be generated as illustrated by the indirect in our carbonation previous studies reaction [9]. betweenCaCO3 and C12 AHA7 geland should CO2 in bethe generated air, which by can the beindirect described carbonation by Equations reaction (5) between and (6) [ C1712].A The7 and occurrence CO2 in the of air, Equation which can(5) be resulted described in the by alkalinity Equations of (5) the and slag (6) suspensions [17]. The occurrence (the final pHof Equation values were (5) resulted 10.42, 10.13, in ◦ ◦ ◦ ◦ the10.05, alkalinity 10.56, andof the 10.02 slag for suspensions the slag suspensions (the final pH treated values at were 20 C, 10.42, 40 C, 10.13, 60 C,10.05, 80 10.56,C, and ◦ and100 10.02C, respectively). for the slag suspensions treated at 20 °C, 40 °C, 60 °C, 80 °C, and 100 °C, respectively).

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1 Metastable hydrates 2 3 4 5 6 7 C4ACH11 8 9 10 11 12 (a) (b) 13 14 Unhydrated slag 15 Calcite 16 17 Metastable hydrates 18 19 C4ACH11 20 21 Al(OH)3 22 23 24 25 (c) (d) 26 27 28 29 30 CCA 31 32 33 C3AH6 34

C3AH6 35 Calcite 36 37 38 39 (e) (f) 40 41 42 43 44 45 C3AHX calcite 46 47 48 49 50 51 52 53 (g) 54

Figure 3. Cont.

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55 (g) calcite 56 57 58 59 Hydrocalumite

5. Conclusions Unhydrated slag 60

61 C3AH6

(h) (i) 64

Figure 3. a × b × c × d Figure 3 FESEM pictures of (a)raw ( ) raw slag slag 2000×, 2000 (b)20H-slag,( ) 20H-slag 2000×, 2000 (c)40H-slag,( ) 40H-slag 2000× 2000 , (d),( 40H-slag) 40H- 65 slag8000× 8000 , (e)60H-slag×,(e) 60H-slag 2000× 2000 , (f)× 60H-slag,(f) 60H-slag 5000×, 5000 (g)80H×,(g slag) 80H 2000×, slag 2000(h)100H-slag×,(h) 100H-slag 2000× and 2000 (i)× 100H-and 66 (slagi) 100H-slag 5000× 5000×. 67

The FESEM image of the 60H-slag (Figure3e) indicates the formation of flake-shaped 68 and cubic hydrates that should be amorphous calcium carboaluminate (CCA) and C3AH6, respectively. In addition, a dense rhombohedral-shaped calcite layer covered part of the slag surface (Figure3f). Therefore, it is assumed that CCA was formed through the reaction between C3AH6 and calcite [18,19]. This also provides an explanation for why the 20H-slag contained large amounts of C4ACH11 but small amounts of calcite formation (the reaction ◦ between C3AH6 and calcite can be described by Equation (7) [19]). After hydration at 80 C, the morphology of the slag changed from separated particles to continuous gel (Figure3g). The flake-shaped gel should be C3AHx, and calcite particles were scattered on the surface of C3AHX in the 80H-slag (Figure3g). For the 100H-slag, flake-shaped hydrocalumite and cubic C3AH6 particles were embedded in the unhydrated slag particles (Figure3h), and a rhombohedral-shaped calcite layer deposited on the edge of the slag particles (Figure3i), similarly to the 40H and 60H slag.

2+ − − Ca12Al14O33 + 33H2O → 12Ca + 14Al(OH)4 + 10OH (5)

2+ − Ca + CO2 + 2Al(OH)4 → CaCO3↓ + 2Al(OH)3↓ + H2O (6)

CaCO3 + Ca3Al2O6·6H2O + 5H2O → 3CaO·Al2O3·CaCO3·11H2O (7)

The BET speciﬁc surface area (SBET) of the slags were listed in Table2. The S BET of the 20H-slag and the 80H-slag was more than two times that of the raw slag, while other pretreated slags demonstrated only a slight SBET increase compared with the raw slag. This should be attributed to the larger amount of ﬂake-shaped hydrates in the 20H-slag and the 80H-slag [20].

Table 2. The BET speciﬁc surface area of the pretreated slags.

Raw Slag 20H-Slag 40H-Slag 60H-Slag 80H-Slag 100H-Slag SBET (m2/g) 4.68 8.32 5.89 6.15 9.40 5.15

Figure4 shows the TG-DSC analysis results of the pretreated slags. The endothermic ◦ peak of around 260–270 C denoted the decomposition of Al(OH)3 and appeared in all slags [19]. This peak was overlapped by the endothermic peak between 280 ◦C and ◦ 325 C, which was attributed to the dehydration of C3AH6 in the 40H-slag, 60H-slag, Processes 2021, 9, 1458 7 of 12

and 80H-slag [18]. These results corresponded well with the XRD and FESEM analysis. ◦ The endothermic peak at 157 C in the 20H-salg indicated the dehydration of C4ACH11 [16], while this peak became broad for the 40H-slag due to the low crystallinity of C4ACH11 [16], as was also reflected in the broad peak of XRD patterns (Figure1). The absence of C 4ACH11 in the other slags can be explained by its instability in temperatures above 40 ◦C[19]. A very broad endothermic peak between 80 and 200 ◦C was observed in the 60H-slag, generated by the dehydration of amorphous CCA [14]. The endothermic peak at 155 ◦C in the 80H-slag represented the dehydration of C3AHx which is close to the dehydration temperatures of CAH10 and C2AH8 [15]. With respect to the 100H-slag, the dehydration of hydrocalumite was reflected in the endothermic peak around 146 ◦C. In general, the dehydration of hydrates mainly occurred over the temperature range of 105–325 ◦C, resulting in significant weight loss. The other significant weight loss region was between 600 and 800 ◦C, which was ascribed to the CaCO3 decomposition. During the hydrothermal process, C12A7 was transformed into calcium aluminates hydrate (CAH), CAC, AH, and CaCO3; therefore, the mass loss ratio of the slags (See Table3) above the temperature range of 105–800 ◦C should be an indicator of C12A7 hydration extent. It may be concluded from the results in Processes 2021, 9, 1458 7 of 12 Table2 that the 20H-slag and the 80H-slag had significantly higher hydration extent than other three slags, in good agreement with the XRD and FESEM analysis.

Figure 4. TG-DSC curves of the pretreated slags. Figure 4. TG-DSC curves of the pretreated slags. Table 3. The mass loss ratio of the pretreated slags between 105 and 800 ◦C. Table 3. The mass loss ratio of the pretreated slags between 105 and 800 °C. Sample Mass Loss Ratio between 105 and 800 ◦C (wt%) Sample Mass Loss Ratio between 105 and 800 °C (wt%) 20H-slag 20.58 20H-slag 20.58 40H-slag 13.80 40H-slag 60H-slag 13.80 15.95 60H-slag 80H-slag 15.95 20.70 80H-slag 100H-slag20.70 15.07 100H-slag 15.07

In conclusion, cubic C3AH6 was a main hydration product for the 40H-slag, 60H-slag, andIn 100H-slag. conclusion, Part cubic of C C3AH3AH6 6could was a react main with hydration CaCO3 productto generate for CCAthe 40H-slag, while the 60H-slag, unreacted and 100H-slag. Part of C3AH6 could react with CaCO3 to generate CCA while the unreacted rhombohedral-shaped CaCO3 layer covered the slag surface, resulting in the hindrance of further hydration for these three slags. By contrast, flake-shaped C4ACഥH11 and C3AHx were the main hydration products for the 20H-slag and the 80H-slag, respectively, and their higher specific surface area may accelerate the carbonation reaction.

3.2. Carbonation Efficiency of Slags Pretreated at Different Temperatures The XRD patterns of the slags after carbonation are shown in Figure 5. After carbonation, the peaks of C3AHx, C4ACഥH11, and hydrocalumite disappeared or showed a significant decrease in intensity while the calcite peaks increased in intensity. This suggests the carbonation of these hydrates. On the contrary, C3AH6 appeared less active in terms of its carbonation, which may be the main cause of the less intense calcite peaks in the car- bonated100H-slag with C3AH6 as the main hydration product (see Figure 1). In addition, residual C4ACഥ H11 peaks were observed, indicating the incomplete carbonation of C4ACഥH11 in the 20H-slag during carbonation. This resulted in the less intense calcite peaks in the carbonated 20H-slag compared with the carbonated 80H-slag.

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rhombohedral-shaped CaCO3 layer covered the slag surface, resulting in the hindrance of further hydration for these three slags. By contrast, ﬂake-shaped C4ACH11 and C3AHx were the main hydration products for the 20H-slag and the 80H-slag, respectively, and their higher speciﬁc surface area may accelerate the carbonation reaction.

3.2. Carbonation Efﬁciency of Slags Pretreated at Different Temperatures The XRD patterns of the slags after carbonation are shown in Figure5. After carbonation, the peaks of C3AHx,C4ACH11, and hydrocalumite disappeared or showed a signiﬁcant decrease in intensity while the calcite peaks increased in intensity. This suggests the carbonation of these hydrates. On the contrary, C3AH6 appeared less active in terms of its carbonation, which may be the main cause of the less intense calcite peaks in the carbonated100H-slag with C3AH6 as the main hydration product (see Figure1). In Processes 2021, 9, 1458 addition, residual C4ACH11 peaks were observed, indicating the incomplete carbonation8 of 12

of C4ACH11 in the 20H-slag during carbonation. This resulted in the less intense calcite peaks in the carbonated 20H-slag compared with the carbonated 80H-slag.

FigureFigure 5. XRD 5. XRD patterns patterns of of (1) (1) carbonated carbonated 20H-slag, 20H-slag, (2 (2)) carbonated 40H-slag,40H-slag, (3)(3) carbonated carbonated 60H-slag, 60H- slag,(4) (4) carbonated carbonated 80H-slag, 80H-slag, and and (5) (5) carbonated carbonated 100H-slag. 100H-slag.

FigureFigure 6 6exhibits exhibits the the FESEMFESEM picturespictures of of the the slags slags after after carbonation. carbonation. For theFor carbonated the car- 20H-slag, ﬂake-shaped C ACH was decomposed and cubic calcite crystals were observ- bonated 20H-slag, flake-shaped4 C411ACഥH11 was decomposed and cubic calcite crystals were observableable (Figure (Figure6a). In 6a). the In carbonated the carbonated 40H-slag 40H-slag and 60H-slag, and 60H-slag, some ofsome the of cubic the calcitecubic calcite crystals were surrounded by unreacted hydrates (Figure6b,c). Larger amounts of cubic calcite crystals were surrounded by unreacted hydrates (Figure 6b,c). Larger amounts of cubic crystals appeared in the carbonated 80H-slag than in the other slags, leading to the break- calcite crystals appeared in the carbonated 80H-slag than in the other slags, leading to the down of continuous C3AHx gel (Figure6d). Moreover, the carbonation products were breakdown of continuous C3AHx gel (Figure 6d). Moreover, the carbonation products covered by a small amount of unreacted C3AHx debris (Figure6d). The microstructure of were covered by a small amount of unreacted C3AHx debris (Figure 6d). The microstruc- the carbonated 100H-slag (Figure6e) was similar to the carbonated 40H-slag and 60H-slag; ture of the carbonated 100H-slag (Figure 6e) was similar to the carbonated 40H-slag and it was composed of unreacted hydration products, unhydrated slag, and some cubic calcite 60H-slag; it was composed of unreacted hydration products, unhydrated slag, and some crystals. In each of the slags, the cubic calcite was generated by the direct reaction of CO cubic calcite crystals. In each of the slags, the cubic calcite was generated by the direct 2 with the hydrates and amorphous AH, as the other reaction product surrounded the cubic reaction of CO2 with the hydrates and amorphous AH, as the other reaction product sur- calcite crystals [11]. In each of the carbonated slags, calcite appeared as non-uniform rounded the cubic calcite crystals [11]. In each of the carbonated slags, calcite appeared as aggregated crystal particles, which indicate direct carbonation [21]. Direct carbonation non-uniform aggregated crystal particles, which indicate direct carbonation [21]. Direct of alkaline slag involved two stages: CO2 dissolution and carbonation reaction [21,22]. carbonationThe simpliﬁed of alkaline direct carbonationslag involved mechanism two stages: of CO slags2 dissolution with hydrothermal and carbonation pretreatment reac- in tionthis [21,22]. study The was simplified summarized direct in Tablecarbonation4. mechanism of slags with hydrothermal pretreatment in this study was summarized in Table 4.

Al(OH)3

C3AH6 Unhydrated slag calcite Al(OH)3 calcite C4ACഥH11

(a) (b)

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Figure 5. XRD patterns of (1) carbonated 20H-slag, (2) carbonated 40H-slag, (3) carbonated 60H- slag, (4) carbonated 80H-slag, and (5) carbonated 100H-slag.

Figure 6 exhibits the FESEM pictures of the slags after carbonation. For the carbonated 20H-slag, flake-shaped C4ACഥH11 was decomposed and cubic calcite crystals were observable (Figure 6a). In the carbonated 40H-slag and 60H-slag, some of the cubic calcite crystals were surrounded by unreacted hydrates (Figure 6b,c). Larger amounts of cubic calcite crystals appeared in the carbonated 80H-slag than in the other slags, leading to the breakdown of continuous C3AHx gel (Figure 6d). Moreover, the carbonation products were covered by a small amount of unreacted C3AHx debris (Figure 6d). The microstructure of the carbonated 100H-slag (Figure 6e) was similar to the carbonated 40H-slag and 60H-slag; it was composed of unreacted hydration products, unhydrated slag, and some cubic calcite crystals. In each of the slags, the cubic calcite was generated by the direct reaction of CO2 with the hydrates and amorphous AH, as the other reaction product surrounded the cubic calcite crystals [11]. In each of the carbonated slags, calcite appeared as non-uniform aggregated crystal particles, which indicate direct carbonation [21]. Direct carbonation of alkaline slag involved two stages: CO2 dissolution and carbonation reac- Processes 2021, 9, 1458 tion [21,22]. The simplified direct carbonation mechanism of slags with hydrothermal pre-9 of 12 treatment in this study was summarized in Table 4.

Al(OH)3

C3AH6 Unhydrated slag calcite Al(OH)3 calcite C4ACഥH11

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(a) (b)

Al(OH)3 calcite Al(OH)3 CCA C3AHx

calcite Unhydrated slag

C3AH6

calcite Al(OH)3 calcite

Hydrocalumite Unhydrated slag

(e)

FigureFigure 6. 6. FESEMFESEM pictures pictures of ( ofa) carbonated (a) carbonated 20H-slag, 20H-slag, (b) carbonated (b) carbonated 40H-slag, 40H-slag, (c) carbonated (c) carbonated 60H- slag,60H-slag, (d) carbonated (d) carbonated 80H-slag, 80H-slag, and (e and) carbonated (e) carbonated 100H-slag. 100H-slag.

TableTable 4. 4. ChemicalChemical reaction reaction equations equations of of the the direct direct carbon carbonationation of of the the pretreated pretreated slags slags in in this this study. study.

StageStage Chemical Chemical Reaction Reaction Equation Equation CO2(g) → CO2(aq) CO2 dissolution CO2(g) → CO2(aq) CO2 dissolutionCO2(aq) + 2OH−(aq) −→ CO32−(aq) 2+− H2O(l) CO2(aq) + 2OH (aq) → CO3 (aq) + H2O(l) Ca3Al2O6·6H2O(s) + 3CO32−(aq) → 3CaCO3(s) + 2Al(OH)3(s) + 6OH−(aq) Ca Al O ·6H O + 3CO 2− → 3CaCO + 2Al(OH) + 6OH− 3 2 6 2 (s) 2− 3 (aq) 3(s) 3(s) − (aq) Ca3Al2O6·xH2O(s) + 3CO3 (aq)2 −→ 3CaCO3(s) + 2Al(OH)3(s) + 6OH (aq)− + (x − Ca3Al2O6·xH2O(s) + 3CO3 (aq) → 3CaCO3(s) + 2Al(OH)3(s) + 6OH (aq) + Carbonation 6)H2O(l) Carbonation (x − 6)H2O(l) 2− 2− 3CaO·Al3CaO·2AlO32·CaCOO3·CaCO3·11H3·11H2O2(s)O +(s) 3CO+ 3CO3 3(aq) →(aq) 4CaCO→ 4CaCO3(s) 3(s)+ 2Al(OH)+ 2Al(OH)3(s)3(s) + + − − 6OH6OH(aq) +(aq) 5H+2O(l) 5H2O(l)

Based on the TG curves of the carbonated slags (Figure 7), CO2 uptake (wt%) was calculated with Equation (1) where m was the dry weight at 325 °C (at this temperature, free water and chemically bound water evaporated). The results of CO2 uptake were listed in Table 5. The CO2 uptake of slags followed this order: 80H-slag (13.66 wt%) > 20H-slag (10.82 wt%) > 60H-slag (9.83 wt%) > 40H-slag (9.21 wt%) > 100H-slag (6.84 wt%), which corresponded well with the XRD and FESEM results. This is attributed to the following reasons: (1) a dense CaCO3 or AH gel layer covered the unhydrated slag surface in the 40H-slag and the 60H-slag, therefore resulting in the hindrance of further hydration and carbonation; (2) flake-shaped hydrates such as C4ACഥH11 in the 20H-slag or C3AHx in the 80H-slag provided a larger reaction surface aera than the cubic C3AH6 and raw slag particles, avoiding calcite and AH gel layer formation on the unreacted hydrates surface. In short, the carbonation efficiency was strongly dependent on the type and morphology of the hydrates of LF slag.

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Based on the TG curves of the carbonated slags (Figure7), CO 2 uptake (wt%) was calculated with Equation (1) where m was the dry weight at 325 ◦C (at this temperature, free water and chemically bound water evaporated). The results of CO2 uptake were listed in Table5. The CO 2 uptake of slags followed this order: 80H-slag (13.66 wt%) > 20H-slag (10.82 wt%) > 60H-slag (9.83 wt%) > 40H-slag (9.21 wt%) > 100H-slag (6.84 wt%), which corresponded well with the XRD and FESEM results. This is attributed to the following reasons: (1) a dense CaCO3 or AH gel layer covered the unhydrated slag surface in the 40H-slag and the 60H-slag, therefore resulting in the hindrance of further hydration and carbonation; (2) ﬂake-shaped hydrates such as C4ACH11 in the 20H-slag or C3AHx in the 80H-slag provided a larger reaction surface aera than the cubic C3AH6 and raw slag particles, avoiding calcite and AH gel layer formation on the unreacted hydrates surface. Processes 2021, 9, 1458 10 of 12 In short, the carbonation efﬁciency was strongly dependent on the type and morphology of the hydrates of LF slag.

Figure 7.7. TGTG curvescurves ofof (1)(1) carbonatedcarbonated 20H-slag,20H-slag, (2)(2) carbonatedcarbonated 40H-slag,40H-slag, (3)(3) carbonatedcarbonated 60H-slag,60H-slag, (4)(4) carbonatedcarbonated 80H-slag,80H-slag, andand (5)(5) carbonatedcarbonated 100H-slag.100H-slag.

TableTable 5.5. COCO22 uptake of pretreated slags.slags.

20H-Slag20H-Slag 40H-Slag 40H-Slag 60H-Slag 60H-Slag 80H-Slag 80H-Slag 100H-Slag 100H-Slag

COCO2 Uptake(wt%)2 Uptake(wt%) 10.82 10.82 9.21 9.21 9.83 9.83 13.66 13.66 6.84 6.84

The maximum CO2 uptake among these five pretreated slags was 13.66 wt% (namely The maximum CO2 uptake among these ﬁve pretreated slags was 13.66 wt% (namely 136.6 g of CO2/1 kg of slag) which was attained by the 80H-slag. Compared with previous 136.6 g of CO2/1 kg of slag) which was attained by the 80H-slag. Compared with previ- ousstudies studies about about wet wetdirect direct carbonation carbonation of steelmaking of steelmaking slags slags conducted conducted under under ambient ambient temperature and pressure (See Table 6), the CO2 uptake of the 80H-slag in this study was temperature and pressure (See Table6), the CO 2 uptake of the 80H-slag in this study was considerableconsiderable andand thethe processprocess waswas attractive.attractive. WithWith anan annualannual (2019–2020)(2019–2020) outputoutput ofof LFLF slagslag of about 5 MT in China, this waste could, under 80 °C◦C hydrothermal hydrothermal pretreatment, capturecapture about 0.67 MT CO2 if the mineral carbonation process is applied in steel plants. about 0.67 MT CO2 if the mineral carbonation process is applied in steel plants.

TableTable 6.6. ComparisonComparison of of current current with with previous previous steelmaking steelmaking slag slag carbonation carbonation studies studies conducted conducted under under ambient temperature and pressure. ambient temperature and pressure.

Slag Type CO2 Uptake (g of CO2/1 kg of Slag) Conditions Reference Slag Type CO2 Uptake (g of CO2/1 kg of Slag)Temperature: Conditions 40 °C Reference Al-rich LF slag 136.6 Temperature:S/W ratio: 1:10 40 ◦C This study Al-rich LF slag 136.6 S/WReaction ratio: time: 1:10 1 h This study ReactionTemperature: time: 20 1°C h EAF steel slag 87 S/W ratio: 1:10 [23] Reaction time: 37 min Temperature: 70 °C BOF steel slag 116.4 S/W ratio: 1:2 [24] Reaction time: 2 h Temperature: 20 °C LF slag 67 S/W ratio: 1:20 [25] Reaction time: 1 h Temperature: 60 °C BOF steel slag 168.32 S/W ratio: 1:30 [26] Reaction time: 10 h Temperature: 20 °C LF slag 56 S/W ratio: 1:5 [27] Reaction time: 70 min Temperature: 60 °C BOF steel slag 215 S/W ratio: 1:5 [28] Reaction time: 3 h

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Table 6. Cont.

Slag Type CO2 Uptake (g of CO2/1 kg of Slag) Conditions Reference Temperature: 20 ◦C EAF steel slag 87 S/W ratio: 1:10 [23] Reaction time: 37 min Temperature: 70 ◦C BOF steel slag 116.4 S/W ratio: 1:2 [24] Reaction time: 2 h Temperature: 20 ◦C LF slag 67 S/W ratio: 1:20 [25] Reaction time: 1 h Temperature: 60 ◦C BOF steel slag 168.32 S/W ratio: 1:30 [26] Reaction time: 10 h Temperature: 20 ◦C LF slag 56 S/W ratio: 1:5 [27] Reaction time: 70 min Temperature: 60 ◦C BOF steel slag 215 S/W ratio: 1:5 [28] Reaction time: 3 h

4. Conclusions This study investigated the temperature of hydrothermal pretreatment on the hydrate formation and carbonation efficiency of Al-rich LF slag at ambient temperature and pressure. The main results are as follows: During hydrothermal pretreatment, cubic C3AH6 was a main hydration product for ◦ ◦ ◦ 40 C, 80 C, and 100 C-pretreated slags while C4ACH11 and C3AHx with flaked shapes were the main hydrates for 20 ◦C and 80 ◦C-pretreated slags, respectively. Rhombohedral- shaped CaCO3 was generated by the reaction between C12A7 in the slag and CO2 in the air; and then CaCO3 reacted with C3AH6 to form flake-shaped CCA. Flake-shaped products presented higher BET specific surface area. In 40 ◦C, 60 ◦C, and 100 ◦C-pretreated slags, a dense CaCO3 layer surrounded the unreacted core of the slag particle, resulting in the hindrance of further C12A7 hydration. Flake-shaped products could provide a lager reaction surface area and avoid the calcite and AH gel layer formation on the surface of the unreacted hydrates. Therefore, 80 ◦C and ◦ 20 C-pretreated slags containing a larger number of flake-shaped hydrates had larger CO2 uptake (13.66 wt% and 10.82 wt%, respectively). Cubic C3AH6 crystal and unhydrated raw slag particles were less inactive for carbonation, resulting in the smaller CO2 uptake for 40 ◦C, 60 ◦C, and 100 ◦C-pretreated slags (9.21 wt%, 9.83 wt% and 6.84 wt%, respectively). In short, the carbonation efficiency of the pretreated slag was strongly associated with the morphology of the hydration products and the hydration extent of LF slag.

Author Contributions: Investigation, Y.H.; resources, G.X.; writing—original draft preparation, G.X.; writing—review and editing, Y.H.; supervision, G.X.; funding acquisition, Y.H. Both authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the Natural Science Foundation of Hunan Province, China (Grant No. 2020JJ4157). Data Availability Statement: The data presented in this study are available on request from the cor- responding author. The data are not publicly available due to patent issues. Acknowledgments: The authors thank the support from the Research Foundation of the Natural Science Foundation of Hunan Province, China (Grant No. 2020JJ4157). Conﬂicts of Interest: The authors declare no conﬂict of interest. Processes 2021, 9, 1458 12 of 12

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